Issues in the Comparison of Ground Gravity with GRACE DataDavid Crossley, Saint Louis U., Dept. Earth & Atmospheric Science, 3507 Laclede Ave., St. Louis MO 63104 , crossley@eas.slu.edu.

Jacques Hinderer, EOST, 5 rue Descartes, Strasbourg 67084, France, Jacques.Hinderer@eost.u-strasbg,fr

Jurgen Neumeyer, Dept. Geodesy and Remote Sensing, GFZ Potsdam, Germany, neum@gfz-potsdam.de

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Figure 2. Individual station gravity residuals after correcting for local tides, nominal atmospheric pressure loading, and IERS polar motion. Amplitudes are in Gal, with time in days since 1 Jan 2002. Vertical grid marks are semi-annual.

Offsets are critical to long period SG observations and are removed with care. The two series from dual sphere instruments (MO, WE) are treated separately and combined only after all corrections. The linear trend contains secular gravity changes seen by AG instruments and the SG instrument drift.

ACKNOWLEDGEMENTSACKNOWLEDGEMENTS

The ground gravity data is from the ICET database, with other data supplied by: Bernd Richter, Herbert Wilmes, Peter

Wolf, Michel van Camp, and Corinna Kroner. Bernard Ducarme assisted in tidal processing. Jean-Paul Boy provided the hydrology loading. For access to the JPL GRACE data and processing, we thank Frank Lemoine

(NASA), and C. K. Shum and S. C. Han (Ohio State). This research is supported by NSF EAR #0409381 and CNRS.

AGU Session ED03 …AGU Session ED03 …

"The availability of sophisticated large-format printers has created a terrible temptation for scientists to present papers with linear storylines inherited from printed publications. Attending the average poster session becomes the daunting equivalent of reading 10 or

20 papers in less than an hour.“

Principal eigenvector for GRACE data

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GGP stations 1997-2001. Stations BE and PO have stopped, and we did not use ME in this study

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GRACE gravity field, truncation n = 20

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Comparison of the first principal eigenvectors and time components that account for 80% variance reduction of the GRACE data and 60% of the SG variability

ABSTRACT AND PROCEDUREABSTRACT AND PROCEDURE

We processed 2 years of SG data from 6 stations in central Europe (Fig. 1) and did an EOF (Empirical Orthogonal Function) analysis to determine the principal

components (PCs) and eigenvectors.

We took GRACE Level 2 monthly satellite solutions, and extracted a portion of the gravity field over the same area as the SG stations. We also did an EOF

analysis of this data.

We compared both gravity field to the predictions of loading from a comprehensive hydrological model.

We computed 3-D atmospheric attraction and deformation as an improvement to our previous empirical admittance -0.3 Gal / hPa (results for only 3 stations

were completed, we do not show them)

OBJECTIVE – TO EXAMINE 3 ISSUESOBJECTIVE – TO EXAMINE 3 ISSUES

Is it possible to compare ground and satellite gravity fields at the precision (Gal) and resolution (10-1000 km) of hydrology? This we have done here.

What is the effect of improving the atmospheric attraction to due a 3-D realistic atmospheric model? This we have done partly.

How should one correct for SG stations that have mass above the gravimeters – thus contributing an additional upwards seasonal effect from soil moisture and snow not seen by the satellite. This we have yet to do.

Station Sensor Offsets removed (Gal) Trend (Gal yr-1)

MB Single sphere 6, from –3.1 to 1.9 Gal 3.86

MC Single sphere 1, at 45.0 Gal 2.31

MO_L Dual sphere, lower 2, from –8.7 to 5.7 Gal 4.83

MO_U Dual sphere, upper 2, from –8.5 to 4.8 Gal 1.95

ST Single sphere none 2.94

VI Single sphere 3, from –1.5 to 4.0 Gal 1.45

WE_L Dual sphere, lower 6, from –41.1 to 5.1 Gal -4.09

WE_U Dual sphere, upper 6, from –43.0 to 7.5 Gal -2.28

LOCAL HYDROLOGYLOCAL HYDROLOGY

The SG data are not corrected for hydrology, which is part of the target signal for GRACE comparisons. Local hydrology

affects SG stations, but if the mass is below the station the local effects should be smoothed by spatial averaging (lots of

instruments).

Comparison between hydrology (soil moisture and snow) loading

in gravity and SG gravity residuals.

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Evolution of first GRACE PC with increasing number of months of data showing stability and downward trend in gravity

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Snapshots of SG gravity field, minimum curvature surface

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